Field electron emission materials and devices

ABSTRACT

A field electron emission material comprises an electrically conductive substrate and, disposed thereon, electrically conductive particles embedded in, formed in, or coated by a layer of inorganic electrically insulating material. A first thickness material is defined between the particle and the environment in which the material is disposed. The dimension of each particle between the first and second thicknesses is significantly greater than each thickness. Upon application of a sufficient electric field, each thickness provides a conducting channel, to afford electron emission from the particles By use of an inorganic insulating material, surprisingly good stability and performance have been obtained. The particles can be relatively small, such that the electron emitting material can be applied to the substrate quite cheaply by a variety of methods, including printing. The material can be used in a variety of devices, including display and illuminating devices.

BACKGROUND OF THE INVENTION

This invention relates to field electron emission materials, and devicesusing such materials.

In classical field electron emission, a high electric field of, forexample, ≈3×10⁹ V m⁻¹ at the surface of a material reduces the thicknessof the surface potential barrier to a point at which electrons can leavethe material by quantum mechanical tunnelling. The necessary conditionscan be realised using atomically sharp points to concentrate themacroscopic electric field. The field electron emission current can befurther increased by using a surface with a low work function. Themetrics of field electron emission are described by the well knownFowler-Nordheim equation.

There is considerable prior art relating to tip based emitters, whichterm describes electron emitters and emitting arrays which utilise fieldelectron emission from sharp points (tips). The main objective ofworkers in the art has been to place an electrode with an aperture (thegate) less than 1 μm away from each single emitting tip, so that therequired high fields can by achieved using applied potentials of 100V orless--these emitters are termed gated arrays. The first practicalrealisation of this was described by C A Spindt, working at StanfordResearch Institute in California (J. Appl. Phys. 39(7), 3504-3505,1968). Spindt's arrays used molybdenum emitting tips which wereproduced, using a self masking technique, by vacuum evaporation of metalinto cylindrical depressions in a SiO₂ layer on a Si substrate.

In the 1970s, an alternative approach to produce similar structures wasthe use of directionally solidified eutectic alloys (DSE). DSE alloyshave one phase in the form of aligned fibres in a matrix of the other.The matrix can be etched back leaving the fibres protruding. Afteretching, a gate structure is produced by sequential vacuum evaporationof insulating and conducting layers. The build up of evaporated materialon the tips acts as a mask, leaving an annular gap around a protrudingfibre.

A further discussion of the prior art is now made with reference toFIGS. 1 and 2 of the accompanying diagrammatic drawings, in which FIG. 1shows basic components of one field electron emission display, and FIG.2 shows the conceptual arrangement of another field electron emissiondisplay.

An important approach is the creation of gated arrays using siliconmicro-engineering. Field electron emission displays utilising thistechnology are being manufactured at the present time, with interest bymany organisations world-wide. FIG. 1 shows basic components of such adisplay in which a field electron emission current is extracted frompoints 1 by applying a positive potential to gate electrodes 2. Theextracted electrons are accelerated by a higher positive potential to apatterned phosphor on conducting strips 3 on a front plate. Pixels areaddressed by energising horizontal and vertical stripes in a crossbararrangement. The device is sealed around the perimeter and evacuated.

A major problem with all point based emitting systems is theirvulnerability to damage by ion bombardment, ohmic heating at highcurrents and the catastrophic damage produced by electrical breakdown inthe device. Making large area devices is both difficult and costly.

In about 1985, it was discovered that thin films of diamond could begrown on heated substrates from a hydrogen-methane atmosphere, toprovide broad area field emitters.

In 1991, it was reported by Wang et al (Electron. Lett., 1991, 27, pp1459-1461) that field electron emission current could be obtained frombroad area diamond films with electric fields as low as 3 MV m⁻¹. Thisperformance is believed to be due to a combination of the negativeelectron affinity of the (111) facets of diamond and the high density oflocalised, accidental graphite inclusions (Xu, Latham and Tzeng:Electron. Lett. 1993, 29, pp 1596-159).

Coatings with a high diamond content can now be grown on roomtemperature substrates using laser ablation and ion beam techniques.However, all such processes utilise expensive capital equipment.

S I Diamond in the USA has described a field electron emission display(FED) that uses as the electron source a material that it calls AmorphicDiamond. The diamond coating technology is licensed from the Universityof Texas. The material is produced by laser ablation of graphite onto asubstrate. FIG. 2 shows the conceptual arrangement in such a display. Asubstrate 4 has conducting strips 5 with Amorphic diamond emittingpatches 6. A front plate 8 has transparent conducting tracks 7 with anapplied phosphor pattern (not shown). Pixels are addressed using acrossbar approach. Negative going waveforms 9 are applied to theconductive strips 5 and positive going waveforms are applied toconductive strips 7. The use of positive and negative going waveformsboth reduces the peak voltage rating for the semiconductors in the driveelectronics and ensures that adjacent pixels are not excited. The deviceis sealed around the perimeter and evacuated.

Turning now to Composite Field Emitters, current understanding of fieldelectron emission from flat metal surfaces shows that active sites areeither metal-insulator-vacuum (MIV) structures formed by embeddeddielectric particles or conducting flakes sitting on the surface oxideof the metal. In both cases, the current comes from a hot electronprocess that accelerates the electrons resulting in quasi-thermionicemission. This is described in the scientific literature (e.g. Latham,High Voltage Vacuum Insulation, Academic Press 1995)

In 1988 (S Bajic and R V Latham, Journal of Physics D Applied Physics,vol. 21 (1988) 200-204), a material that made practical use of the abovemechanism was described. The composite material creates a high densityof metal-insulator-metal-insulator-vacuum (MIMIV) emitting sites. Thecomposite had conducting particles dispersed in an epoxy resin. Thecoating was applied to the surface by standard spin coating techniques.

The emission process is believed to occur as follows. Initially theepoxy resin forms a blocking contact between the particles and thesubstrate. The voltage of a particle will rise to the potential of thehighest equipotential it probes--this has been called the antennaeffect. At a certain applied voltage, this will be high enough to createan electro-formed conducting channel between the particle and thesubstrate. The potential of the particle then flips rapidly towards thatof the cathode. The residual charge above the particle then produces ahigh electric field which creates a second electro-formed channel and anassociated MIV hot electron emission site. After this switch-on process,reversible field emitted currents can be drawn from the site. Thecurrent density/electric field performance of this material isequivalent to broad area diamond emitters produced by the much moreexpensive laser ablation process.

Bajic and Latham worked with resin-carbon composites. Although theyconsidered the use of alternative materials, these were alwayscomposites with resin (supra and Inst Phys Conf Ser No. 99; Sectzon 4-pp101-104, 1989). Epoxy resins provided materials that were convenient towork with, particularly in view of their adhesive properties making itconvenient to place and hold particles where desired, in composite orlayered structures. However, materials such as those produced by Bajicand Latham have tended to have poor stability, and not to worksatisfactorily in sealed-off vacuum devices.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention aim to provide costeffective broad area field emitting materials and devices that utilisesuch materials. The materials may be used in devices that include: fieldelectron emission display panels; high power pulse devices such aselectron MASERS and gyrotrons; crossed-field microwave tubes such asCFAs; linear beam tubes such as klystrons; flash x-ray tubes; triggeredspark gaps and related devices; broad area x-ray sources forsterilisation; vacuum gauges; ion thrusters for space vehicles; particleaccelerators; ozonisers; and plasma reactors.

According to one aspect of the present invention, there is provided afield electron emission material comprising an electrically conductivesubstrate and, disposed thereon, at least one electrically conductiveparticle embedded in, formed in, or coated by a layer of inorganicelectrically insulating material to define a first thickness of theinsulating material between the particle and the substrate and a secondthickness of the insulating material between the particle and theenvironment in which the material is disposed, the dimension of saidparticle between said thicknesses, in a direction normal to thesubstrate, being at least twice each said thickness.

The use of an inorganic electrically insulating material has providedunexpected advantages. Such materials do not naturally suggestthemselves as insulators in this context since, as compared to materialssuch as epoxy resins, they are relatively difficult to work with.However, in preferred embodiments of the invention, emitting materialsof surprisingly good stability and performance have been achieved, byusing electrically conductive particles in an inorganic electricallyinsulating material.

Preferably, said dimension of said particle is at least 10 times greaterthan each said thickness.

Preferably, said dimension of said particle is at least 100 timesgreater than each said thickness.

In a preferred example, said thickness may be of the order of 10 nm (100Å) and said particle dimension of the order of 100 μm.

There may be provided a substantially single layer of said conductiveparticles each having their longest dimension in the range 0.1 μm to 400μm.

Preferably, said inorganic insulating material comprises a materialother than diamond.

Preferably, said inorganic insulating material comprises a glass, leadbased glass, glass ceramic, melted glass or other glassy material,ceramic, oxide ceramic, oxidised surface, nitride, nitrided surface, orboride ceramic.

Said inorganic insulating material may comprise undoped diamond.

By "undoped diamond" is meant diamond that has not undergone intentionaldoping to facilitate the passage of current.

The or each said electrically conductive particle may comprise agraphite inclusion that has been deliberately engineered in thin-filmdiamond as said inorganic insulating material.

The or each said electrically conductive particle may comprise a fibrechopped into a length longer than its diameter.

The or each said electrically conductive particle may be substantiallysymmetrical.

The or each said electrically conductive particle may be ofsubstantially rough-hewn cuboid shape.

A field electron emission material as above may comprise a plurality ofsaid conductive particles, preferentially aligned with their longestdimension substantially normal to the substrate.

A field electron emission material as above may comprise a plurality ofconductive particles having a mutual spacing in the range 5 to 15 timestheir longest dimension.

A field electron emission material as above may comprise a structure inwhich said layer of inorganic electrically insulating material comprisesan electrically insulating matrix and there are provided a plurality ofsaid electrically conductive particles as an array of conductive fibressubstantially supported in said insulating matrix with exposed fibreends substantially co-planar with the insulating matrix, and the exposedfibre ends and co-planar matrix substantially covered with anelectrically insulating sublayer.

Said structure may be bonded by means of an electrically conductivemedium to said electrically conductive substrate.

Preferably, the fibres have a length in the range 1 μm to 2 mm and adiameter in the range 0.5 μm to 100 μm.

Preferably, the inter-fibre spacing is in the range 5 to 15 times thefibre length.

The fibre array may be formed from a slice of a directionally solidifiedeutectic material.

Preferably, a respective said insulating sub-layer is provided on eachof two opposite faces of said structure.

Preferably, the thickness of the or each insulating sub-layer is in therange 5 nm (50 Å) to 2 μm.

The or each insulating sub-layer may comprise a glass, glass ceramic,ceramic, oxide ceramic, nitride, boride ceramic or diamond.

Preferably, the conductivity of the conducting particle is such that apotential drop caused by the emission current passing through theparticle is sufficient to reduce the electric field at the emissionpoint of the particle by an amount that controls the emission current.

Preferably, said particle comprises, or at least some of said particlescomprise, silicon carbide, tantalum carbide, hafnium carbide, zirconiumcarbide, the Magneli sub-oxides of titanium, semiconducting silicon,III-V compounds and II-VI compounds.

Said particle may comprise a gettering material and have at least oneportion which is not covered by said layer of insulating material, inorder to expose said portion to said environment.

According to another aspect of the present invention, there is provideda method of forming a field electron emission material according to anyof the preceding aspects of the invention, comprising the step ofdisposing the or each said electrically conductive particle on saidelectrically conductive substrate with the or each said electricallyconductive particle embedded in, formed in, or coated by said layer ofinorganic electrically insulating material.

Preferably, said electrically conductive particle(s) and/or inorganicelectrically insulating material are applied to said electricallyconductive substrate by a printing process.

Said electrically conductive particle(s) and/or inorganic electricallyinsulating material may be applied to said electrically conductivesubstrate in a photosensitive binder.

A method as above may include the step of sintering or otherwise joiningtogether a mixture of larger and smaller particles, the larger particlescomprising a plurality of said conductive particles and the smallerparticles forming said layer of inorganic insulating material. Theinsulating material may then comprise glass ceramic, ceramic, oxideceramic, nitride, boride or diamond.

A method as above may include the steps of applying sequentially to thesubstrate an insulating film, conductive particle layer and furtherinsulating film. The insulating material may then comprise a ceramic,oxide ceramic, oxide, nitride, boride or diamond.

A method as above may include the steps of applying an insulatingcoating directly onto each of a plurality of said conductive particlesand then fixing the coated particles to the substrate by a glassymaterial or braze. The insulating material may then comprise glass,glass ceramic, ceramic, oxide ceramic, oxide, nitride, boride ordiamond.

Said layer of inorganic insulating material may comprise a porousinsulator and said method may include the step of filling the pores ofthe porous insulator with a conductive material to provide a pluralityof said conductive particles.

A method as above may include the step of forming two outer sub-layersof inorganic insulating material on opposite faces of said porousinsulator, so that said porous insulator comprises a middle sub-layerbetween said two outer sub-layers of inorganic insulating material.

Where the particle is a part-coated gettering material as mentionedabove, the method may include the steps of bonding a plurality of saidparticles to said substrate, and only partly coating said particles withsaid insulating material, by means of a roller. Alternatively, themethod may include the steps of bonding a plurality of said particles tosaid substrate, and evaporating said insulating material from a sourcesuch that the evaporated material impinges on the surface of theparticles at an angle, thereby only partly coating said particles withsaid insulating material.

The invention extends to a field electron emission material produced byany of the above methods.

According to a further aspect of the present invention, there isprovided a field electron emission device comprising a field electronemission material according to any of the preceding aspects of theinvention.

A field electron emission device as above may comprise a substrate withan array of emitter patches of said field electron emission material,and a control electrode with an aligned array of apertures, whichelectrode is supported above the emitter patches by an insulating layer.

Said apertures may be in the form of slots.

A field electron emission device as above may comprise a plasma reactor,corona discharge device, silent discharge device or ozoniser.

A field electron emission device as above may comprise an electronsource, electron gun, electron device, x-ray tube, vacuum gauge, gasfilled device or ion thruster.

The field electron emission material may supply the total curre it foroperation of the device.

The field electron emission material may supply a starting, triggeringor priming current for the device.

A field electron emission device as above may comprise a display device.

A field electron emission device as above may comprise a lamp.

Preferably, said lamp is substantially flat.

A field electron emission device as above may comprise an electrodeplate supported on insulating spacers in the form of a cross-shapedstructure.

The field electron emission material may be applied in patches which areconnected in use to an applied cathode voltage via a resistor.

Preferably, said resistor is applied as a resistive pad under eachemitting patch.

A respective said resistive pad may be provided under each emittingpatch, such that the area of each such resistive pad is greater thanthat of the respective emitting patch.

Preferably, said emitter material and/or a phosphor is/are coated uponone or more one-dimensional array of conductive tracks which arearranged to be addressed by electronic driving means so as to produce ascanning illuminated line.

Such a field electron emission device may include said electronicdriving means.

The environment may be gaseous, liquid, solid, or a vacuum.

A field electron emission device as above may include a getteringmaterial within the device.

Preferably, said gettering material is affixed to the anode.

Said gettering material may be affixed to the cathode. Where the fieldelectron emission material is arranged in patches, said getteringmaterial may be disposed within said patches.

In one embodiment of the invention, a field electron emission device asabove may comprise an anode, a cathode, spacer sites on said anode andcathode, spacers located at some of said spacer sites to space saidanode from said cathode, and said gettering material located on saidanode at others of said spacer sites where spacers are not located.

In the context of this specification, the term "spacer site" means asite that is suitable for the location of a spacer to space an anodefrom a cathode, irrespective of whether a spacer is located at thatspacer site.

Preferably, said spacer sites are at a regular or periodic mutualspacing.

In a field electron emission device as above, said cathode may beoptically translucent and so arranged in relation to the anode thatelectrons emitted from the cathode impinge upon the anode to causeelectro-luminescence at the anode, which electro-luminescence is visiblethrough the optically translucent cathode.

It will be appreciated that the electrical terms "conducting" and"insulating" can be relative, depending upon the basis of theirmeasurement. Semiconductors have useful conducting properties and,indeed, may be used in the present invention as conducting particles. Inthe context of this specification, the or each said conductive particlehas an electrical conductivity at least 10² times (and preferably atleast 10³ or 10⁴ times) that of the inorganic electrically insulatingmaterial.

In the context of this specification, the term "inorganic electricallyinsulating material" includes inorganic materials with organicimpurities and, in particular, includes thin film-diamond.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention, and to show how embodimentsof the same may be carried into effect, reference will now be made, byway of example, to FIGS. 3 to 19 of the accompanying diagrammaticdrawings, in which:

FIG. 1 shows basic components of one field electronic emission display;

FIG. 2 shows the conceptual arrangement of another field electronemission display;

FIG. 3a shows one example of an improved field electron emissionmaterial;

FIG. 3b illustrates an alternative material to that of FIG. 3a;

FIG. 4 shows a gated array using an improved field electron emissionmaterial;

FIG. 5 illustrates steps in an alternative method of producing animproved field electron emission material;

FIG. 6a illustrates a coated conductive particle;

FIG. 6b illustrates one example of an improved field electron emissionmaterial using coated conductive particles as shown in FIG. 6a;

FIG. 6c illustrates another example of an improved field electronemission material using coated conductive particles as shown in FIG. 6a;

FIG. 7a shows a field electron emission display using an improved fieldelectron emission material;

FIGS. 7b and 7c are detail views showing modifications of parts of thedisplay of FIG. 7a;

FIG. 8a shows a flat lamp using an improved field electron emissionmaterial and FIG. 8b shows a detail thereof;

FIG. 9 illustrates a further method of producing an improved fieldelectron emission material;

FIG. 10a shows an alternative, high performance embodiment of theinvention;

FIG. 10b shows a detail of the embodiment of FIG. 10a;

FIG. 11 shows a variant of the embodiment of FIGS. 10a and 10b;

FIG. 12a illustrates a self-buffering effect in a conductive particle;

FIG. 12b shows measured voltage-current characteristics for emitterswith graphite and silicon carbide patches;

FIG. 13 shows two pixels in a colour display, utilising a triode systemwith a control electrode;

FIG. 14 shows a display in which spacers are replaced with getteringmaterial;

FIG. 15 shows a display in which getter patches are disposed withinemitter patches;

FIG. 16 illustrates a getter particle used to make a MIMIV emitter;

FIGS. 17a and 17b illustrate respective methods of making a structurewith a porous insulating layer;

FIG. 18 illustrates a high conversion efficiency field emission lampwith light output through the emitter layer; and

FIG. 19 shows a sub-pixel of an electrode system, where the gate toemitter spacing has been reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrated embodiments of the invention provide materials basedupon the MIMIV emission process with improved performance and usability,together with devices that use such materials.

Heating effects in electro-formed channels limit the mean currentavailable from MIV and MIMIV emitters. Furthermore, the increasedtemperatures degrade the material, changing its properties and causinginstability or catastrophic failure.

The temperature rise in a channel (ΔT) is described by equations of theform

    ΔT=2β.sub.2 E.sub.o I/πKε.sub.r log(l/α)

Where: β₂ is the field enhancement factor due to the geometry of thechannel; E_(o) is the gap field; I is the current flowing in thechannel; K is the thermal conductivity of the medium; ε_(r) is thedielectric constant of the medium; a is the radius of the channel; and 1is the length of the channel.

FIG. 3a shows an improved material with conducting particles 11 in aninorganic matrix such as a glass 12 on a conducting substrate 13. Thisstructure increases the thermal conductivity of the matrix 12approximately four times, as compared to conventional materials. Ofequal importance is the increased thermal stability of the inorganicmatrix. These two factors combine to produce a material that can supplya significantly higher current, before channel heating causesinstability or failure. An inorganic matrix also eliminates high vapourpressure organic materials, enabling the material to be used insealed-off vacuum devices. For insulating substrates 13, a conductinglayer 14 is applied before coating. The conducting layer 14 may beapplied by a variety of means including, but not limited to, vacuum andplasma coating, electroplating, electroless plating and ink basedmethods.

The standing electric field required to switch on the electro-formedchannels is determined by the ratio of particle height 16 and thethickness of the matrix in the region of the conducting channels 15. Fora minimum switch on field, the thickness of the matrix 12 at theconducting channels should be significantly less than the particleheight. The conducting particles would typically be in, although notrestricted to, the range 0.1 μm to 400 μm, preferably with a narrow sizedistribution.

Structures of this form may be produced (FIG. 3b) by sintering a mixtureof large conducting particles 50 mixed with significantly smallerinsulating particles 51. Examples of suitable insulating materials are:glass ceramics, oxide ceramics, nitrides, borides although a wide rangeof other materials may be used. For high current applications, highthermal conductivity materials such as beryllia and aluminium nitridemay be used to improve performance.

The structure may also be produced by melting a glass with good flowproperties, such as a lead glass, with the particles. Such a structureis shown in FIG. 3a. Using glassy materials, the thickness of thechannel regions may be controlled by varying the time/temperatureprofile during firing.

To enable the material to be applied in a controlled manner, it can beformulated as an ink with a no-residue binder similar to materials usedfor hybrid electronic circuits. Such a binder may be photosensitive toenable patterning by photo-lithography. Using an ink so prepared, theemitter may be applied in patterns using hybrid microcircuit techniquessuch as screen printing. Alternative application methods may be usedincluding, but not limited to, offest lithography, ink-jet printing,electrostatic coating (optionally with photo-resist), Xerography, brushcoating, electrophoresis, plasma or flame spraying and sedimentation.Thus, the field emitting material may be printed onto a suitablesubstrate, opening up new opportunities for economical fabrication ofdisplays, etc.

One suitable ink can be formulated from a mixture of a spin-on glassmaterial, particles (optionally with a narrow size distribution) adispersing agent and a binder. Such spin-on glass materials aretypically based on polysiloxanes and are used extensively in thesemiconductor industry. However, spin-on glasses based upon otherchemical compounds may be used.

FIG. 5 shows an alternative method of producing desired structures. Aconducting substrate 24, which could be produced by over-coating aninsulating substrate, has an insulating film 25 deposited upon it. Sucha film may be produced by, but not limited to, vacuum or plasma basedcoating, spin coating and in situ growth by chemical reaction or anodicprocesses. Conducting particles 26 are then deposited as a layer on theinsulating film 25 by a dry coating technique such as, but not limitedto, electrostatic coating, Xerography or brush coating. During thisstage, electrostatic or magnetic fields may be used to align theparticles to achieve optimum electric field enhancement. An insulatingcoating 27 is then deposited over the particles by typically a vacuum orplasma based process.

FIG. 6a shows a conductive particle 28 pre-coated with an insulatingfilm 29 by methods which include: vacuum or plasma based coating,chemical vapour deposition, anodic processes. A plurality of such coatedparticles 30 are then fixed to the substrate 31 by a glassy material orbraze alloy 32, as shown in FIG. 6a. Examples of acceptable materialsare lead glasses and reactive braze alloys such as Zr-Cu eutectic.

In the alternative material shown in FIG. 6b, a plurality of coatedparticles 30 are fixed directly to the substrate 31. In this case, theinsulating film 29 is of a material suitable to be fixed directly to thesubstrate 31-egg lass.

FIG. 9 shows an alternative approach in which a substrate 70 is firstcoated with an insulating film 71. A much thicker porous insulating film72 is then applied. A conducting material 73 is then infiltrated intothe pores by chemical reaction, electroplating or another method.Finally, a second thin insulating film 74 is applied.

In all the above-described embodiments of the invention, there is anoptimum density of conducting particles that prevents thenearest-neighbour particles screening the electric field at the tip of agiven particle. For spherical particles, the optimumparticle-to-particle spacing is approximately 10 times the particlediameter.

Intentionally engineered structures like those in FIG. 3a are asubstantial improvement upon relatively small, randomly created graphiteinclusions in thin film diamond. An important feature is that the ratioof particle height 16 to insulator barrier thickness 15 is much greaterthan in diamond films. As a result, the increased antenna effectsignificantly reduces the switch-on field.

To facilitate even switch-on of emitting sites, symmetrical particles,such as those of a rough hewn cuboid shape are preferred.

Alternatively, precision fibres, such as carbon fibre or fine wire, maybe chopped into lengths somewhat longer than their diameter. Thetendency of these fibre segments will be to lie down (especially duringspin coating) with the fibre axis parallel to the substrate such thatthe diameter of the fibre determines the antenna effect.

Particles of the correct morphology (e.g. glass microspheres) but notcomposition may be over coated with a suitable material by a wide rangeof processes including sputtering.

A primary purpose of preferred embodiments of the invention is toproduce emitting materials with low cost and high manufacturability.However, for less cost-sensitive applications, the very high thermalconductivity that may be achieved means that intentionally engineeredstructures, using diamond as the insulator can provide materials thatcan deliver the highest mean currents before catastrophic failure of theelectro-formed channels.

FIG. 4 shows a gated array using one of the improved field electronemission materials. Emitter patches 19 are formed on a substrate 17 onwhich a conducting layer 18 is deposited, if required, by a process suchas screen printing. A perforated control or gate electrode 21 isinsulated from the substrate 17 by a layer 20. Typical dimensions areemitter patch diameter (23) 100 μm; gate electrode-substrate separation(22) 20 μm. A positive voltage on the gate electrode 21 controls theextraction of electrons from the emitter patches 19. The electrons 53are then accelerated into the device 52 by a higher voltage 54. Thefield electron emission current may be used in a wide range of devicesincluding: field electron emission display panels; high power pulsedevices such as electron MASERS and gyrotrons; crossed-field microwavetubes such as CFAs; linear beam tubes such as klystrons; flash x-raytubes; triggered spark gaps and related devices; broad area x-raysources for sterilisation; vacuum gauges; ion thrusters for spacevehicles and particle accelerators.

It is known that an MIV process emits electrons with energies of a fewelectron volts. The mean free path of such electrons in a solid issurprisingly long. Thus, if the emitter material has a thin (eg lessthan 100 nm=1000 Å) conducting layer deposited on the surface, and isbiased a few hundred volts positive with respect to the substrate, MIMIVprocesses will occur. With such a thin conducting layer, the majority ofemitted electrons will pass through the conducting layer into theenvironment. Such a conducting layer may be used as a control electrodeto modulate the emitted current in a wide range of devices. Such aconducting layer may be used in many embodiments of the invention.

An alternative high performance embodiment of the invention is shown inFIGS. 10a and 10b. A regular array of fibres 80 is embedded in aninsulating matrix 81. The length of the fibres is typically a fewhundred microns. Such structures can be fabricated or may be foundnaturally in directionally solidified ceramic-metal eutectic systems.The inter-fibre spacing (82) is typically several times the fibrelength.

The composite so formed is cut into slices and each face is preferably(although optionally) polished. The two polished faces are then coatedwith an inorganic insulating film 83 of a controlledthickness--typically around 10 nm (100 Å). The film 83 may be of, butnot limited to, glass, glass ceramic, ceramic, oxide ceramic, nitride,boride ceramic or diamond and may be deposited by vacuum coating, ionbeam processing, chemical vapour deposition, laser ablation or otherappropriate method.

The sandwich structure so formed is then bonded to a substrate 85 usinga conducting layer 84. Such a bond could be formed using an active metalbrazing alloy. Alternatively, the surface to be bonded may be metallisedprior to brazing using a non-reactive alloy.

The array can provide all the current for a device or act as a triggerfor plasma processes (eg spark gaps) or starting current for sourcesthat use secondary emission multiplication (eg magnetron injectionguns).

If the material of FIGS. 10a and 10b is for use in a non-vacuumenvironment, the insulating material 81 may comprise a relativelylow-grade material, such as a cheap resin simply to support the fibres80, provided that the insulating films 83 are of an inorganic material.

In the variant of FIG. 11, fibres 90 protrude above the level of theinsulating material 81, and are covered by a respective film 91 ofinorganic insulating material. Otherwise, the embodiment is generallysimilar to those described above with reference to FIGS. 10a and 10b.

FIG. 7 shows a field emission based upon a diode arrangement using oneof the above-described materials--eg the material of FIG. 9. A substrate33 has conducting tracks 34 which carry emitting patches 35 of thematerial. A front plate 38 has transparent conducting tracks 39 runningacross the tracks 34. The tracks 39 have phosphor patches or stripes.The two plates are separated by an outer ring 36 and spacers 43. Thestructure is sealed by a material 37 such as a solder glass. The deviceis evacuated either through a pumping tube or by fusing the solder glassin a vacuum furnace.

Pixels are addressed by voltages 41, 42 applied in a crossbar fashion.The field emitted electrons excite the phosphor patches. A drive systemconsisting of positive and negative going waveforms both reduces thepeak voltage rating for the semiconductors in the drive electronics, andensures that adjacent pixels are not excited. Further reductions in thevoltage swing needed to turn pixels on can be achieved by DC biasingeach electrode to a value just below that at which the field electronemission current becomes significant. A pulse waveform is thensuperimposed on the DC bias to turn each pixel on: voltage excursionsare then within the capability of semiconductor devices.

An alternative approach to the diode arrangement is to utilise a triodesystem with a control electrode. FIG. 13, which depicts two pixels in acolour display, shows one embodiment of this approach. For pictorialsimplicity only two pixels are shown. However the basic structure shownmay be scaled up to produce large displays with many pixels. A cathodesubstrate 120 has conducting tracks 121 coated onto its surface toaddress each line in the display. Such tracks may be deposited by vacuumcoating techniques coupled with standard lithographic techniques wellknown to those skilled in the art; by printing using a conducting ink;or many other suitable techniques. Patches 122 of the emitting materialdescribed above are disposed, using the methods described previously,onto the surface of the tracks to define sub-pixels in a Red-Green-Bluetriad. Dimension "P" 129 is typically in, although not limited to, therange 200 μm (micrometer) to 700 μm. Alternatively, although lessdesirable, the emitting material may be coated over the whole displayarea. An insulating layer 123 is formed on top of the conducting tracks121. The insulating layer 123 is perforated with one or more aperturesper pixel 124 to expose the emitting material surface, such aperturesbeing created by printing or other lithographic technique. Conductingtracks 125 are formed on the surface of the insulator to define a gridelectrode for each line in the colour triad. The dimensions of theapertures 124 and the thickness of the insulator 123 are chosen toproduce the desired value of transconductance for the triode system soproduced. The anode plate 126 of the display is supported on insulatingspacers 128. Such spacers may be formed on the surface by printing ormay be prefabricated and placed in position. For mechanical stability,said prefabricated spacers may be made in the form of a cross-shapedstructure. A gap filling material, such as a glass fritt, may be used tofix both the spacer in position at each end and to compensate for anydimensional irregularities. Red, green and blue phosphor patches orstripes 127 are disposed on the inside surface of the anode plate. Thephosphors are either coated with a thin conducting film as is usual incathode ray tubes or, for lower accelerating voltages, the inside of theanode plate has deposited on it a transparent conducting layer such as,but not limited to, indium tin oxide. The interspace between the cathodeand anode plates is evacuated and sealed.

A DC bias is applied between conducting strips 121 and the conductingfilm on the anode. The electric field so produced penetrates through thegrid apertures 124 and releases electrons from the surface by fieldemission from the MIMIV field emission process described earlier. The DCvoltage is set lower than required for full emission thus enabling aline to be addressed by pulsing one of the tracks 121 negative withrespect to the others to a value that gives the current for peakbrightness. The grid tracks 125 are biased negative with respect to theemitter material to reduce the current to its minimum level when thetracks 121 are in their negative pulsed (line addressed) state. Duringthe line period all grid tracks are pulsed positively up to a value thatgives the desired current and hence pixel brightness. Clearly otherdriving schemes may be used.

To minimise the cost of the drive electronics, gate voltage swings of afew tens of volts are needed. To meet this specification, the aperturesin the gate electrode structures shown in FIG. 13 become quite small.With circular apertures, this results in many emitting cells persub-pixel. An alternative arrangement for such small structures is toelongate the small emitting cells into slots.

FIG. 19 shows one sub-pixel of such an electrode system, where the gateto emitter spacing 180 has been reduced to a few micrometres. The gate181 and insulator layer 182 have slots 183 in them, exposing theemitting material.

Although a colour display has been described, it will be understood bythose skilled in the art that an arrangement without the three-partpixel may be used to produce a monochrome display.

To ensure a long life and stable operating characteristics a high vacuummust be maintained in the device. It has been normal in the art ofelectron tubes to use getters to adsorb gas desorbed from the walls andother internal structures. One location for gettering materials in fieldemitting displays is around the perimeter of the display panel on thosesides where there are no electrical feedthroughs. It is well known tothose skilled in the art that this location becomes far from ideal asthe panel size increases. This is because of the low gas flowconductance between the centre and the edge of the panel that resultsfrom the long distances and sub-millimetre clearances between thepanels. Calculations show that for panels greater than a 250 mm diagonaldimension this conductance drops to a level where the getter systembecomes ineffective. U.S. Pat. No. 5,223,766 describes two methods ofovercoming this problem. One method involves a cathode plate with anarray of holes leading into a back chamber with larger clearances anddistributed getters. The other method is to make the gate electrode of abulk gettering material such as zirconium. Although both methods work inprinciple there are distinct practical problems with them.

In the perforated cathode plate approach, the perforations in thecathode plate must be small enough to fit within the spaces between thepixels. To avoid visible artefacts this limits their diameter to amaximum of 125 micrometers for television and rather less for computerworkstations. The cost of drilling millions of ˜100 micrometers holes in1 mm to 2 mm thick glass, the obvious material for the cathode plate, islikely to be prohibitive. Furthermore, the resulting component will beextremely fragile: a problem that will increase with increasing paneldimensions.

In order to be effective at room temperature, bulk getters must have avery high surface area. This is usually achieved by forming a sinteredparticulate layer. The gate electrode in a field emitting display sitsin a strong accelerating DC field. It is clear from the field emittersystems described herein that such particulate getter layers are likelyto provide a significant number of field emitting sites. Such sites willemit electrons continuously exciting one or more of the phosphor patchesin the vicinity to produce a visible defect in the display.

Turning now to the display shown in FIG. 13 three methods are describedby which a distributed getter system may be incorporated into thestructure. Whilst such methods are described in the context of thisdisplay using the emitter systems described herein, it will beunderstood that the techniques may be used with displays using otheremitter systems.

A suitable location for a particulate getter material such that it doesnot cause spurious emission is the anode plate. At the anode thestanding electric field totally suppresses electron emission. In a fieldemission display the cathode and anode plates are subjected to largeforces by the external atmospheric pressure. To prevent distortion andfracture, spacers are disposed between the plates. Said spacers areincorporated into the pixel structure. In order to minimise visibleartefacts, obscuring lines are printed onto the anode plate to hide thespacer contact areas. Whilst it is usual to repeat the spacers with theperiodicity of the pixels, such an arrangement results in significantmechanical over-design. It is thus possible to reduce the frequency ofspacers and to locate gettering material on the anode plate behind theobscuring lines. FIG. 14 shows one embodiment with a cathode plate 130and anode plate 131 supported on spacers 133. The spacer contact areason the anode plate are masked by obscuring lines 134. In this embodimentspacers are removed from two potential locations and replaced withgettering material 135. Suitable gettering materials are finely dividedGroup IVa metals such as Zirconium and proprietary gettering alloys suchas those produced by SAES Getters of Milan. Such gettering material maybe in the form of particles bonded to the anode plate by brazing orglass fritts. Equally it may be directly deposited as a porous layer bya wide range of methods including thermal spraying and vapour coating inan inert scattering gas. Clearly other methods may be devised. Saidgetters are activated during fritt sealing of the structure, passivatedupon exposure to air and then reactivated during the bakeout phase ofvacuum processing.

An alternative method is to locate gettering material within the emitterareas such that any field emitted electrons are modulated along withintentionally emitted electrons and such that spurious electrons augmentthose from the emitter patches. FIG. 15 shows one embodiment of this inwhich getter patches 170 are disposed within emitter patches 171 suchthat spurious electrons only excite the phosphor patches 172 whenaddressed by the drive electronics.

FIG. 16 shows another approach in which a getter particle, or cluster ofparticles, is used to make a MIMIV emitter as described above. Theemission mechanism does not require the particle to be entirely coatedin insulator since the critical areas are the contact point with thesubstrate and the emitting area towards the top of the particle. In thisembodiment a particle 140 is fixed to a substrate 141 by an insulatingmaterial 142. The upper portions of the particle are coated with aninsulating layer 143. The compositions of the insulating materials 142and 143 are as described herein. This arrangement leaves an area ofexposed gettering material 144.

Alternatively the insulating layer may coat the entire particle but besubstantially porous. FIG. 17 shows two method of making suchstructures. FIG. 17a shows particles 151 bonded to a substrate 150 by aninsulating material 152. The upper portions of the particles are coatedwith an insulator 153 by means of a roller 154. Material is dispensedonto the roller by a system 155. An alternative method, shown in FIG.17b, is to take a substrate with particles bonded as described above andto vacuum evaporate an insulating material 161 from a point or linesource 162 such that the evaporated material impinges on the surface atan oblique angle. Shadowing ensures that only the top and one side ofthe particles are coated. To ensure a uniform insulator thickness thesubstrate is traversed past the source.

A problem with all field electron emission displays is in achievinguniform electrical characteristics from pixel to pixel. One approach isto use electronics that drive the pixels in a constant current mode. Analternative approach that achieves substantially the same objective isto insert a resistor of appropriate value between the emitter and aconstant voltage drive circuit. This may be external to the device.However, in this arrangement, the time constant of the resistor and thecapacitance of the conducting track array places a limit on the ratethat pixels can be addressed. Forming the resistor in situ between theemitter patch and the conducting track enables low impedance electronicsto be used to rapidly charge the track capacitance, giving a muchshorter rise time. Such an in situ resistive pad 44 is shown in FIG. 7b.The resistive pad may be screen printed onto the conducting track 34,although other coating methods may be used. In some embodiments, thevoltage drop across the resistive pad 44 may be sufficient to causevoltage breakdown across its surface 45. To prevent breakdown, anoversize resistive pad 46 may be used to increase the tracking distance,as illustrated in FIG. 7c.

The mechanism of operation of the MIMIV emitters previously describedoffers an alternative method of buffering the emission to resistivepads. In the publication S Bajic and R V Latham, Journal of Physics DApplied Physics, vol. 21 200-204 it is proposed that, after "switch-on",current flows from the substrate via an electroformed channel, into theparticle and is then emitted into the vacuum from a further conductingchannel at another point on the particle. This mechanism is showndiagrammatically in FIG. 12a. It can be seen from this diagram that theemitted current 113 must flow through the particle 110 to be emittedinto the vacuum. Between the two conducting channels 112 is the internalresistance of the particle 114. Current flowing from the substrate 109causes a potential drop across the particle that depends on itsresistivity. This potential drop reduces the field at the top of theparticle which, in turn, limits the rate of rise of current withelectric field. Thus, a self-buffering effect is achieved.

FIG. 12b shows measured voltage-current characteristics for emitterswith graphite 115 and silicon carbide 116 particles. Over a large rangethe emitter using silicon carbide particles displays a linear, ratherthan Fowler-Nordheim-like, voltage-current characteristic. Thevoltage-emission current characteristic is determined by the resistanceof the particle rather than the properties of the conducting channels.Process control of particle size and resistivity is far easier than theadventitiously electro-formed channels. An important benefit of this isgreater uniformity and substantially reduced temporal fluctuations ofemission compared to emitters with graphite particles.

Modelling shows that the potential drop across the particle at themaximum current shown is in excess of 100 volt. The two examples shownare extremes with resistivities differing by at least 1000:1. Bychoosing particles with intermediate resistivities, a trade-off can bemade between the reduced control voltage swing of theFowler-Nordheim-like characteristic and the stability of the heavilybuffered linear characteristic. An optimum choice can be made for eachapplication.

FIG. 8a shows a flat lamp using one of the above-described materials.Such a lamp may be used to provide backlighting for liquid crystaldisplays, although this does not preclude other uses such as roomlighting.

The lamp comprises a back plate 60 which may be made of a metal that isexpansion matched to a light transmitting front plate 66. If the backplate is an insulator, then a conducting layer 61 is applied. Theemitting material 62 is applied in patches. To force the system towardsequal field emitted current per emitting patch, and hence produce auniform light source, each patch is electrically connected to the backplate via a resistor. Such a resistor can be readily formed by anelectrically resistive pad 69, as shown in FIG. 8b. As in FIG. 7c, theresistive pad may have a larger area than the emitting patch, to inhibitvoltage breakdown across its thickness. A more cost-effectivealternative to resistive patches is to use the self-buffering materialsdescribed above. The front plate 66 has a transparent conducting layer67 and is coated with a suitable phosphor 68. The two plates areseparated by an outer ring 63 and spacers 65. The structure is sealed bya material 64 such as a solder glass. The device is evacuated eitherthrough a pumping tube or by fusing the solder glass in a vacuumfurnace. A DC voltage of a few kilovolts is applied between the backplate 60 or the conducting layer 61 and the transparent conductingcoating 67. Field emitted electrons bombard the phosphor 68 and producelight. The intensity of the lamp may be adjusted by varying the appliedvoltage.

For some applications, the lamp may be constructed with addressablephosphor stripes and associated electronics to provide a scanning linein a way that is analogous to a flying spot scanner. Such a device maybe incorporated into a hybrid display system.

Although field emission cathodoluminescent lamps as described aboveoffer many advantages over those using mercury vapour (such as cooloperation and instant start), they are intrinsically less efficient. Onereason for this is the limited penetration of the incident electronsinto the phosphor grains compared with that for ultraviolet light from amercury discharge. As a result, with a rear electron excited phosphor,much of the light produced is scattered and attenuated in its passagethrough the particles. If light output can be taken from the phosphor onthe same side onto which the electron beam impinges, the luminousefficiency may be approximately doubled. FIG. 18 shows an arrangementthat enables this to be achieved.

In FIG. 18 a glass plate 170 has an optically transparent electricallyconducting coating 171 (for example, tin oxide) onto which is formed alayer of MIMIV emitter 172 as described herein. This emitter isformulated to be substantially optically translucent and, beingcomprised of randomly spaced particles, does not suffer from the Moirepatterning that the interference between a regular tip array and thepixel array of an LCD would produce. Such a layer may be formed with,although not limited to, polysiloxane based spin-on glass as theinsulating component. The coated cathode plate described above issupported above an anode plate by spacers 179 and the structure sealedand evacuated in the same manner as the lamp shown in FIG. 8a. The anodeplate 177 which may be of glass, ceramic, metal or other suitablematerial has disposed upon it a layer of a electroluminescent phosphor175 with an optional reflective layer 176, such as aluminium, betweenthe phosphor and the anode plate. A voltage 180 in the kilovolt range isapplied between the conducting layer 171 and the anode plate 177 (or inthe case of insulating materials a conducting coating thereon). Fieldemitted electrons 173 caused by said applied voltage are accelerated tothe phosphor 175. The resulting light output passes through thetranslucent emitter 172 and transparent conducting layer 171. Anoptional Lambertian or non-Lambertian diffuser 178 may be disposed inthe optical path.

Embodiments of the invention may employ thin-film diamond with graphiteinclusions that are optimized to meet the requirements of theinvention--for example, by aligning such inclusions, making them ofsufficient size and density, etc. In the manufacture of thin-filmdiamond, the trend in the art has been emphatically to minimize graphiteinclusions, whereas, in embodiments of the invention, such inclusionsare deliberately included and carefully engineered.

An important feature of preferred embodiments of the invention Is theability to print an emitting, pattern, thus enabling complexmulti-emitter patterns, such as those required for displays, to becreated at modest cost. Furthermore, the ability to print enableslow-cost substrate materials, such as glass to be used; whereasmicro-engineered structures are typically built on high-cost singlecrystal substrates. In the context of this specification, printing,means a process that places or forms an emitting material in a definedpattern. Examples of suitable processes are: screen printing,Xerography, photolithography, electrostatic deposition, spraying oroffset lithography.

Devices that embody the invention may be made in all sizes, large andsmall. This applies especially to displays, which may range from asingle pixel device to a multi-pixel device, from miniature tomacro-size displays.

What is claimed is:
 1. A field electron emission material comprising an electrically conductive substrate and, disposed thereon, at least one electrically conductive particle embedded in, formed in, or coated by a layer of inorganic electrically insulating material to define a first thickness of the insulating material between the particle and the substrate and a second thickness of the insulating material between the particle and the environment in which the material is disposed, the dimension of said particle between said thicknesses, in a direction normal to the substrate, being at least twice each said thickness.
 2. A field electron emission material according to claim 1, wherein said dimension of said particle is at least 10 times greater than each said thickness.
 3. A field electron emission material according to claim 2, wherein said dimension of said particle is at least 100 times greater than each said thickness.
 4. A field electron emission material according to claim 1, wherein there is provided a substantially single layer of said conductive particles each having their longest dimension in the range 0.1 μm to 400 μm.
 5. A field electron emission material according to claim 1, wherein said inorganic insulating material comprises a material other than diamond.
 6. A field electron emission material according to claim 5, wherein said inorganic insulating material comprises a glass, lead based glass, glass ceramic, melted glass or other glassy material, ceramic, oxide ceramic, oxidised surface, nitride, nitrided surface, or boride ceramic.
 7. A field electron emission material according to claim 1, wherein said inorganic insulating material comprises undoped diamond.
 8. A field electron emission material according to claim 1, wherein the or each said electrically conductive particle comprises a graphite inclusion that has been deliberately engineered in thin-film diamond as said inorganic insulating material.
 9. A field electron emission material according to claim 1, wherein the or each said electrically conductive particle comprises a fibre chopped into a length longer than its diameter.
 10. A field electron emission material according to claim 1, wherein the or each said electrically conductive particle is substantially symmetrical.
 11. A field electron emission material according to claim 10, wherein the or each said electrically conductive particle is of substantially rough-hewn cuboid shape.
 12. A field electron emission material according to claim 1, comprising a plurality of said conductive particles, preferentially aligned with their longest dimension substantially normal to the substrate.
 13. A field electron emission material according to claim 1, comprising a plurality of conductive particles having a mutual spacing in the range 5 to 15 times their longest dimension.
 14. A field electron emission material according to claim 1, comprising a structure in which said layer of inorganic electrically insulating material comprises an electrically insulating matrix and there are provided a plurality of said electrically conductive particles as an array of conductive fibres substantially supported in said insulating matrix with exposed fibre ends substantially co-planar with the insulating matrix, and the exposed fibre ends and co-planar matrix substantially covered with an electrically insulating sub-layer.
 15. A field electron emission material according to claim 14, wherein said structure is bonded by means of an electrically conductive medium to said electrically conductive substrate.
 16. A field electron emission material according to claim 14, wherein the fibres have a length in the range 1 μm to 2 mm and a diameter in the range 0.5 μm to 100 μm.
 17. A field electron emission material according to claim 14, wherein the inter-fibre spacing is in the range 5 to 15 times the fibre length.
 18. A field electron emission material according to claim 14, wherein the fibre array is formed from a slice of a directionally solidified eutectic material.
 19. A field electron emission material according to claim 14, wherein a respective said insulating sub-layer is provided on each of two opposite faces of said structure.
 20. A field electron emission material according to claim 14, wherein the thickness of the or each insulating sub-layer is in the range 5 nm (50 Å) to 2 μm.
 21. A field electron emission material according to claim 14, wherein the or each insulating sub-layer comprises a glass, glass ceramic, ceramic, oxide ceramic, nitride, boride ceramic or diamond.
 22. A field electron emission material according to claim 1, wherein the conductivity of the conducting particle is such that a potential drop caused by the emission current passing through the particle is sufficient to reduce the electric field at the emission point of the particle by an amount that controls the emission current.
 23. A field electron emission material according to claim 1, wherein said particle comprises, or at least some of said particles comprise, silicon carbide, tantalum carbide, hafnium carbide, zirconium carbide, the Magneli sub-oxides of titanium, semiconducting silicon, III-V compounds and II-VI compounds.
 24. A field electron emission material according to claim 1, wherein said particle comprises a gettering material and has at least one portion which is not covered by said layer of insulating material, in order to expose said portion to said environment.
 25. A method of forming a field electron emission material according to claim 1, comprising the step of disposing the or each said electrically conductive particle on said electrically conductive substrate with the or each said electrically conductive particle embedded in, formed in, or coated by said layer of inorganic electrically insulating material.
 26. A method according to claim 25, wherein said electrically conductive particle(s) and/or inorganic electrically insulating material are applied to said electrically conductive substrate by a printing process.
 27. A method according to claim 26, wherein said electrically conductive particle(s) and/or inorganic electrically insulating material are applied to said electrically conductive substrate in a photosensitive binder.
 28. A method according to claim 25, including the step of sintering or otherwise joining together a mixture of larger and smaller particles, the larger particles comprising a plurality of said conductive particles and the smaller particles forming said layer of inorganic insulating material.
 29. A method according to claim 28, wherein the insulating material comprises glass ceramic, ceramic, oxide ceramic, nitride, boride or diamond.
 30. A method according to claim 25, including the steps of applying sequentially to the substrate an insulating film, conductive particle layer and further insulating film.
 31. A method according to claim 30, wherein the insulating material comprises a ceramic, oxide ceramic, oxide, nitride, boride or diamond.
 32. A method according to claim 25, 26 or 27, including the steps of applying an insulating coating directly onto each of a plurality of said conductive particles and then fixing the coated particles to the substrate by a glassy material or braze.
 33. A method according to claim 32, wherein the insulating material comprises glass, glass ceramic, ceramic, oxide ceramic, oxide, nitride, boride or diamond.
 34. A method according to claim 25, wherein said layer of inorganic insulating material comprises a porous insulator and said method includes the step of filling the pores of the porous insulator with a conductive material to provide a plurality of said conductive particles.
 35. A method according to claim 34, including the step of forming two outer sub-layers of inorganic insulating material on opposite faces of said porous insulator, so that said porous insulator comprises a middle sub-layer between said two outer sub-layers of inorganic insulating material.
 36. A method according to claim 25, including the steps of bonding a plurality of said particles to said substrate, and only partly coating said particles with said insulating material, by means of a roller.
 37. A method according to claim 25, including the steps of bonding a plurality of said particles to said substrate, and evaporating said insulating material from a source such that the evaporated material impinges on the surface of the particles at an angle, thereby only partly coating said particles with said insulating material.
 38. A field electron emission material produced by a method according to claim
 25. 39. A field electron emission device comprising a field electron emission material according to claim
 1. 40. A field electron emission device according to claim 39, comprising a substrate with an array of emitter patches of said field electron emission material.
 41. A field electron emission device according to claim 40, further comprising a control electrode with an aligned array of apertures, which electrode is supported above the emitter patches by an insulating layer.
 42. A field electron emission device according to claim 41, wherein said apertures are in the form of slots.
 43. A field electron emission device according to claim 39, included in a plasma reactor, corona discharge device, electroluminescent device or display, silent discharge device, ozoniser, electron source, electron gun, electron device, x-ray tube, vacuum gauge, gas filled device or ion thruster.
 44. A field electron emission device according to claim 39, wherein the field electron emission material supplies the total current for operation of the device.
 45. A field electron emission device according to claim 39, wherein the field electron emission material supplies a starting, triggering or priming current for the device.
 46. A field electron emission device according to claim 39, comprising a display device.
 47. A field electron emission device according to claim 39, comprising a lamp.
 48. A field electron emission device according to claim 47, wherein said lamp is substantially flat.
 49. A field electron emission device according to claim 39, comprising an electrode plate supported on insulating spacers in the form of a cross-shaped structure.
 50. A field electron emission device according to claim 39, wherein the field electron emission material is applied in patches which are connected in use to an applied cathode voltage via a resistor.
 51. A field electron emission device according to claim 50, wherein said resistor is applied as a resistive pad under each emitting patch.
 52. A field electron emission device according to claim 51, wherein a respective said resistive pad is provided under each emitting patch, and the area of each such resistive pad is greater than that of the respective emitting patch.
 53. A field electron emission device according to claim 39, wherein said emitter material and/or a phosphor is/are coated upon one or more one-dimensional array of conductive tracks which are arranged to be addressed by electronic driving means so as to produce a scanning illuminated line.
 54. A field electron emission device according to 53, including said electronic driving means.
 55. A field electron emission device according to claim 39, wherein said environment of said material is a vacuum.
 56. A field electron emission device according to claim 39, including a gettering material within the device.
 57. A field electron emission device according to claim 56, wherein said gettering material is affixed to the anode.
 58. A field electron emission device according to claim 56, wherein said gettering material is affixed to the cathode.
 59. A field electron emission device according to claim 58, wherein the field electron emission material is arranged in patches and said gettering material is disposed within said patches.
 60. A field electron emission device according to claim 56, comprising an anode, a cathode, spacer sites on said anode and cathode, spacers located at some of said spacer sites to space said anode from said cathode, and said gettering material located on said anode at others of said spacer sites where spacers are not located.
 61. A field electron emission device according to claim 60, wherein said spacer sites are at a regular or periodic mutual spacing.
 62. A field electron emission device according to claim 39, wherein said cathode is optically translucent and so arranged in relation to the anode that electrons emitted from the cathode impinge upon the anode to cause electro-luminescence at the anode, which electro-luminescence is visible through the optically translucent cathode. 